Push through a bulletproof revolving door in a nondescript building in a dreary patch of the former East Berlin and you enter the control center for Vattenfall Europe Transmission, the company that controls northeastern Germany’s electrical grid. A monitor displaying a diagram of that grid takes up most of one wall. A series of smaller screens show the real-time output of regional wind turbines and the output that had been predicted the previous day. Germany is the world’s largest user of wind energy, with enough turbines to produce 22,250 megawatts of electricity. That’s roughly the equivalent of the output from 22 coal plants–enough to meet about 6 percent of Germany’s needs. And because Vattenfall’s service area produces 41 percent of German wind energy, the control room is a critical proving ground for the grid’s ability to handle renewable power.

Like all electrical grids, the one that Vattenfall manages must continually match power production to demand from homes, offices, and factories. The challenge is to maintain a stable power supply while incorporating elec­tricity from a source as erratic as wind. If there’s too little wind-generated power, the company’s engineers might have to start up fossil-fueled power plants on short notice, an inefficient process. If there’s too much, it could overload the system, causing blackouts or forcing plants to shut down.

The engineers have few options, however. The grid has a limited ability to shunt extra power to other regions, and it has no energy-storage capacity beyond a handful of small facilities that pump water into uphill reservoirs and then release it through turbines during periods of peak demand. So each morning, as offices and factories switch their power on, the engineers must use wind predictions to help decide how much electricity conventional plants should start producing.

But those predictions are far from perfect. As more and more wind turbines pop up in Germany, so do overloads and shortages caused by unexpected changes in wind level. In 2007, ­Vattenfall’s engineers had to scrap their daily scheduling plans roughly every other day to reconfigure electricity supplies on the fly; in early 2008, such changes became necessary every day. Power plants had to cycle on and off inefficiently, and the company had to make emergency electricity purchases at high prices. Days of very high wind and low demand even forced the Vattenfall workers to quickly shut the wind farms down.

Reporter's Notebook: David Talbot

Vattenfall’s problems are a preview of the immense challenges ahead as power from renewable sources, mainly wind and solar, starts to play a bigger role around the world. To make use of this clean energy, we’ll need more transmission lines that can transport power from one region to another and connect energy-­hungry cities with the remote areas where much of our renewable power is likely to be generated. We’ll also need far smarter controls throughout the distribution system–technologies that can store extra electricity from wind farms in the batteries of plug-in hybrid cars, for example, or remotely turn power-hungry appliances on and off as the energy supply rises and falls.

If these grid upgrades don’t happen, new renewable-power projects could be stalled, because they would place unacceptable stresses on existing electrical systems. According to a recent study funded by the European Commission, growing electricity production from wind (new facilities slated for the North and Baltic Seas could add another 25,000 megawatts to Germany’s grid by 2030) could at times cause massive overloads. In the United States, the North American Electric Reliability Corporation, a nongovernmental organization set up to regulate the industry after a huge 1965 blackout, made a similar warning in November. “We are already operating the system closer to the edge than in the past,” says the group’s president, Rick Sergel. “We simply do not have the transmission capacity available to properly integrate new renewable resources.”